The semiconductor industry is now forming copper interconnects between two metal layers in a semiconductor wafer by a damascene process. In a damascene process, openings are formed in a nonconductive layer (also called “dielectric layer”) followed by inlay of copper to form metal lines and/or vias. The damascene process is used to form copper interconnects because traditional plasma etch techniques cannot be used to etch copper (e.g. because copper does not form a volatile by-product of the type formed by aluminum). Moreover, a traditional dielectric layer of SiO2 is being replaced with one or more materials having a lower dielectric constant k. Such new processes are used in fabrication of memory devices as well as logic devices.
FIG. 1A illustrates, in a cross-sectional view, a wafer 100 that is undergoing conventional fabrication, e.g. for formation of via holes through a dielectric layer. Specifically, as shown in FIG. 1A, wafer 100 includes a metal layer 101, which may or may not be patterned into lines (also called “traces”). In addition, wafer 100 also includes a dielectric layer 102 (also called “interlayer dielectric” or “ILD”) that is located over metal layer 101. A photo-resist layer 103 is also shown in FIG. 1A, which has been deposited on dielectric layer 102, and is exposed at certain sites to form holes 103A-103N.
The bottoms of holes 103A-103N (FIG. 1B) are used (with layer 103 as a mask) to etch away dielectric layer 102, up to underlying metal layer 101, thereby to form a corresponding number of via holes 102A-102N that pass all the way through layer 102. The just-described process of forming through holes 102A-102N sometimes leaves residue 104 in a hole 102I, which may, for example, be a chemical or copper oxide. Presence of residue may increase the resistance of a via 105I (FIG. 1C) when it is formed in hole 102I, or affect reliability of via 105I (as compared to other vias 105A and 105N).
Even in the absence of residue, vias 105A-105N in holes 102A-102N may not be formed uniformly, e.g. if corresponding holes 102A-102N are not formed of identical diameters. A via that is too large in diameter may become shorted to an adjacent structure, while a via that is too small in diameter may have too much resistance, or sometimes the via may not be formed uniformly throughout the depth of the via hole.
U.S. Pat. No. 6,054,868 granted to Borden, et al. on Apr. 25, 2000 (and incorporated by reference herein in its entirety) teaches that conductivity of a dielectric layer that is located underneath a conductive layer may be measured by: (1) focusing a heating beam on the heated a region (also called “heated region”) of the conductive layer (2) modulating the power of the heating beam at a predetermined frequency that is selected to be sufficiently low to ensure that at least a majority (preferably all) of the generated heat transfers out of the heated region by diffusion, and (3) measuring the power of a probe beam that is (a) reflected by the heated region, and (b) modulated in phase with modulation of the heating beam. Diffusion of heat occurs in the just-described method, by conduction under steady state conditions, eliminating the creation of a thermal wave as described in U.S. Pat. No. 5,228,776. Note that the dielectric layer described in U.S. Pat. No. 6,054,868 is (a) unpatterned and (b) located underneath the conductive layer.
U.S. Pat. No. 6,040,936 granted to Kim is incorporated by reference herein in its entirety. This patent discloses a metal film having a periodic array of sub-wavelength-diameter holes provided therein, and a supporting layer. At least a portion of the supporting layer has a selectively variable refractive index, the selectively variable refractive index portion being substantially adjacent to the metal film such that the metal film and the supporting layer comprise a perforated metal film unit. Selective variation of the refractive index of the selectively variable refractive index portion modulates the intensity of the light transmitted through the perforated metal film unit without substantially changing the direction of the light. Note that U.S. Pat. No. 6,040,936 describes the sub-wavelength-diameter holes as being formed in a metal film which is not a dielectric film. Moreover, the structure by Kim is not disclosed as being formed during fabrication of integrated circuit (IC) dies in a semiconductor wafer. Also, as the perforated layer is metal, one would expect that there would be a strong signal from that layer that would overwhelm any signal from the bottoms of the holes.
See also U.S. Pat. No. 6,734,968 granted to Wang on May 11, 2004 that is incorporated by reference herein in its entirety. This patent describes two phase modulators or polarizing elements employed to modulate the polarization of an interrogating radiation beam before and after the beam has been modified by a sample to be measured. Radiation so modulated and modified by the sample is detected and up to 25 harmonics may be derived from the detected signal. The up to 25 harmonics may be used to derive ellipsometric and system parameters, such as parameters related to the angles of fixed polarizing elements, circular deattenuation, depolarization of the polarizing elements and retardances of phase modulators. The above-described self-calibrating ellipsometer may be combined with another optical measurement instrument such as a polarimeter, a spectroreflectometer or another ellipsometer to improve the accuracy of measurement and/or to provide calibration standards for the optical measurement instrument. The self-calibrating ellipsometer as well as the combined system may be used for measuring sample characteristics such as film thickness and depolarization of radiation caused by the sample.